1. Field of the Invention
The present invention relates to an apparatus for fabricating porous glass preforms by depositing glass particles onto a deposition base material that are generated by a flame hydrolysis or oxidation reaction using a burner, and more particularly, to a structure of a reaction vessel used for synthesizing the glass particles.
2. Description of the Related Art
Optical fibers are fabricated by reducing the diameter of a large diameter glass preform into a glass rod having a diameter appropriate for fiber-drawing and then drawing the glass rod. The large diameter glass preform is obtained by heat treating a porous glass preform fabricated with the VAD (Vapor phase axial deposition) method or OVD (Outside Vapor Deposition) method so as to sinter and make transparent the glass.
Recently, larger porous glass preforms requires larger fabricating devices and large volume gas needed to manufacture the preforms. The larger fabricating devices in turn require larger installation area, leading to higher cost. If the devices are not made larger, lower equipment cost and smaller installation area is available, and fabricating costs can be controlled, but the increase in thermal load to a reaction vessel would be problematic.
The increase in heat caused by the enlargement of a porous glass preform can increase thermal stress to the reaction vessel, damage the reaction vessel, and cause a serious effect on the reaction vessel's life. When the reaction vessel is damaged, small fragments of plate materials and thermal insulation materials constituting the reaction vessel may float in atmosphere in the reaction vessel and be mixed into soot bodies, resulting in the increase in transmission loss and the break of the final product, an optical fiber.
In order to prolong the reaction vessel's life, it is necessary to reduce heat to which the reaction vessel is subjected. However, the reduction of combustion gas for cutting the heat leads to the decrease in the density of the soot bodies, resulting in damages of the soot bodies during fabricating process or transportation. Furthermore, the decrease in the density will thicken the outer diameter of the soot bodies and require much larger devices in the successive process.
The Japanese Patent Laid-Open No. 2001-010823 discloses a method for water-cooling a reaction vessel as a way of preventing damages of the reaction vessel without enlarging it. However, the method requires a water-cooling mechanism and can cause a new problem of high cost due to the additional cooling water facility and of additional spaces due to the enlargement of a device. The Japanese Patent No. 3674315 discloses a method for preventing damages of the reaction vessel by adding a portion to the reaction vessel itself for releasing stress concentration. The Japanese Patent Laid-Open No. 2003-192356 discloses another method for preventing damages of the reaction vessel by fastening a part of the vessel on a floor and enabling other parts of the reaction vessel to slide to release stress concentration due to the device' heat expansion. The last method is effective for reaction vessels in which any members are not welded to each other . However, with this method, reaction vessels composed of members made by welding a plurality of metal members may crack at the weakest weld when fixed at one location and extended and contracted widely.